In circular knitting machines for hosiery or the like, at the end of a knitting process performed with the thread dispensed by a corresponding thread guide at a feed or drop of the machine, the thread guide is moved into a position in which it prevents the needles from engaging the thread and the thread is cut.
The portion of thread that lies between the corresponding thread guide and the end formed by the cut is kept in a position that allows to resume knitting with that thread.
Generally, thread guides are arranged around the needle cylinder just above its upper end, and in order to prevent the needles from engaging the thread the thread dispensing end of said thread guides is raised.
At the upper end of the needle cylinder, just below the dispensing end of the thread guides, when such end is lowered to feed the thread to the needles, there is a circular cutter, which is arranged coaxially to the needle cylinder and has a slightly smaller diameter than the cylindrical surface traced by the needles as a consequence of the rotary motion of the needle cylinder about its own axis. Said cutter is provided with a plurality of teeth and generally rotates at the same speed as the needle cylinder. When a thread guide is raised in order to prevent the needles from engaging the thread and the needles that have engaged said thread are lowered into the needle cylinder, the portion of thread that lies between the thread guide and the last needle that engaged said thread is engaged by a tooth of the cutter and is drawn until said tooth of the cutter encounters a fixed abutment or blade, which cuts the thread.
In the region between the thread guide and the blade, above the needle cylinder, within the cylindrical surface traced by the needles, there is an aspirator port. In practice, the portion of thread that lies between the corresponding thread guide and the tooth of the cutter that has engaged it, before cutting, is placed below the aspirator port, so that after cutting the cut end of said thread portion is aspirated into the aspirator port. In this manner, the portion of thread that lies between the thread guide and the cut end is retained and kept tensioned, so that when the corresponding thread guide is lowered again said portion of thread descends between two contiguous needles, ensuring that said thread is engaged correctly by the first needle that is moved to knit at the feed being considered in order to knit with said thread.
When the thread is elastically extensible and is fed in a pre-tensioned condition, the retention performed by the aspirator port may be insufficient, and the thread, after cutting, might contract and return toward the dispensing end of the thread guide due to elastic reaction, to the point of sliding out of said thread guide, making it impossible to subsequently resume knitting with that thread.
In these cases, and possibly in the case of particular knitting requirements, downstream of the thread guides of a feed along the direction of rotation of the needle cylinder with respect to the thread guides there are thread gripping devices, which are designed to perform a firmer and more reliable retention of the thread when it is cut.
One of the thread gripping devices currently used in circular knitting machines or hosiery knitting machines comprises a supporting element, which is fixed to the structure of the machine that supports the thread guides proximate to a feed or drop. Said supporting element supports at least one elastic lamina element and a thread locking lever, which is pivoted to the supporting element and can rotate in a scissor-like fashion with respect to the elastic lamina element in order to catch the thread directly before it is cut and grip it between the locking lever and the elastic lamina element.
The locking lever and the elastic lamina element are arranged on planes that are substantially radial with respect to the needle cylinder and are located above the needle cylinder.
The locking lever has, on its lower side, which faces the upper end of the needle cylinder, a lug that forms a shoulder that is directed toward the axis of the needle cylinder.
In order to grip the thread, the locking lever is turned about its own pivoting axis, on its plane of arrangement, which lies substantially radially with respect to the needle cylinder, so that its lower end moves transversely to the path of the thread in order to engage said thread by means of said shoulder and move the thread toward the axis of the needle cylinder and grip it between said locking lever and the elastic lamina element.
In the use of these devices, a certain difficulty has been observed in gripping and retaining small-diameter elastically extensible threads that are fed in a pre-tensioned condition.
In this case, the locking lever and the elastic lamina element are in fact unable to ensure correct gripping of the thread, since said thread, owing to its small diameter and to the tension to which it is subjected, tends to escape from the grip of the locking lever and of the elastic lamina element and to return toward the dispensing end of the thread guide that dispenses it.
An event of this kind is a significant problem, since the thread might slide out of the thread guide and therefore become unavailable when knitting with that thread resumes. In this case, the manual intervention of an operator is required in order to reinsert the thread in the thread guide. If the operator is not available immediately, the machine stops, with a consequent production loss.
On the other hand, the thrust of the elastic lamina element against the locking lever cannot be increased beyond a certain limit without the danger of damaging the thread.